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Systematic Analysis of Cations - Practice Questions & MCQ

Edited By admin | Updated on Sep 18, 2023 18:35 AM | #JEE Main

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Analysis of Cations is considered one of the most asked concept.
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Analysis of Cations

Preliminary Examination of the Salt for Identification of Cation:

Colour Test Observe the colour of the salt carefully, which may provide useful information about the cations.

Colour	Cations Indicated
Light green, Yellow, Brown	$Fe^{2+}, Fe^{3+}$
Blue	$Cu^{2+}$
Bright green	$Ni^{2+}$
Blue, Red, Voilet, Pink	$Co^{2+}$
Light pink	$Mn^{2+}$

Dry Heating Test
(i) Take about 0.1 g of the dry salt in a clean and dry test tube.
(ii) Heat the above test tube for about one minute and observe the colour of the residue when it is hot and also when it becomes cold.
Observation of changes gives indications about the presence of cations, which may not be taken as conclusive evidence.

Colour when cold	Colour when hot	Inference
Blue	White	$Cu^{2+}$
Green	Dirty white or yellow	$Fe^{2+}$
White	Yellow	$Zn^{2+}$
Pink	Blue	$Co^{2+}$

Flame Test:
The chlorides of several metals impart characteristic colour to the flame because they are volatile in non-luminous flame. This test is performed with the help of a platinum wire as follows :
(i) Make a tiny loop at one end of a platinum wire.
(ii) To clean the loop dip it into concentrated hydrochloric acid and hold it in a non-luminous flame
(iii) Repeat step (ii) until the wire imparts no colour to the flame.
(iv) Put 2-3 drops of concentrated hydrochloric acid on a clean watch glass and make a paste of a small quantity of the salt in it.
(v) Dip the clean loop of the platinum wire in this paste and introduce the loop in the non-luminous (oxidising) flame
(vi) Observe the colour of the flame first with the naked eye and then through a blue glass and identify the metal ion.

Colour of the flame Observed by naked eye	Colour of the flame Observed through blue glass	Inference
Green flame with blue centre	Same colour as observed without glass	$Cu^{2+}$
Crimson red	Purple	$Sr^{2+}$
Apple green	Bluish green	$Ba^{2+}$
Brick red	Green	$Ca^{2+}$

Borax Bead Test
This test is employed only for coloured salts because borax reacts with metal salts to form metal borates or metals, which have characteristic colours.
(i) To perform this test make a loop at the end of the platinum wire and heat it in a flame till it is red hot.
(ii) Dip the hot loop into borax powder and heat it again until borax forms a colourless transparent bead on the loop. Before dipping the borax bead in the test salt or mixture, confirm that the bead is transparent and colourless. If it is coloured this means that, the platinum wire is not clean. Then make a fresh bead after cleaning the wire.
(iii) Dip the bead in a small quantity of the dry salt and again hold it in the flame.
(iv) Observe the colour imparted to the bead in the non - luminous flame as well as in the luminous flame while it is hot and when it is cold.
(v) To remove the bead from the platinum wire, heat it to redness and tap the platinum wire with your finger.

Heating in Oxidising (non luminous) flame		Heating in Reducing (luminous) flame		Inference
Colour in cold	Colour in hot	Colour in cold	Colour in hot	Ion
Blue	Green	Red Opaque	Colourless	$Cu^{2+}$
Reddish Brown	Violet	Grey	Grey	$Ni^{2+}$
Light Violet	Light Violet	Colourless	Colourless	$Mn^{2+}$
Yellow	Yellowish brown	Green	Green	$Fe^{3+}$

Wet Tests for Identification of Cations

The cations indicated by the preliminary tests given above are confirmed by systematic analysis. The first essential step is to prepare a clear and transparent solution of the salt. This is called original solution. It is prepared as follows:

Preparation of Original Solution (O.S.):
To prepare the original solution, the following steps are followed one after the other in a systematic order. In case the salt does not dissolve in a particular solvent even on heating, try the next solvent. The following solvents are tried:

Take a little amount of the salt in a clean boiling tube and add a few mL of distilled water and shake it. If the salt does not dissolved, heat the content of the boiling tube till the salt completely dissolves.
If the salt is insoluble in water as detailed above, take fresh salt in a clean boiling tube and add a few mL of dil.HCl to it. If the salt is insoluble in cold, heat the boiling tube till the salt is completely dissolved.
If the salt does not dissolve either in water or in dilute HCl even on heating, try to dissolve it in a few mL of conc. HCl by heating.
If salt does not dissolve in conc. HCl, then dissolve it in dilute nitric acid.
If salt does not dissolve even in nitric acid then a mixture of conc. HCl and conc. HNO3 (3:1 ratio). This mixture is called aqua regia. A salt not soluble in aqua regia is considered to be an insoluble salt.

GROUP ANALYSIS:

Analysis of Zero group cation ( $NH_4^+$ ion):

Take 0.1 g of salt in a test tube and add 1-2 mL of NaOH solution to it and heat. If there is a smell of ammonia, this indicates the presence of ammonium ions. Bring a glass rod dipped in hydrochloric acid near the mouth of the test tube. White fumes are observed.
Pass the gas through Nessler’s reagent. Brown precipitate is obtained.

Chemistry of Confirmatory Tests for $NH_4^+$ ion
Ammonia gas evolved by the action of sodium hydroxide on ammonium salts reacts with hydrochloric acid to give ammonium chloride, which is visible as dense white fume.

$\mathrm{(NH_{4})_{2}SO_{4}\: +\: 2NaOH\: \rightarrow \: Na_{2}SO_{4}\: +\: 2NH_{3}\: +\: 2H_{2}O}$

$\mathrm{NH_{3}\: +\: HCl\: \rightarrow \: NH_{4}Cl}$

On passing the gas through Nessler’s reagent, a brown colouration or a precipitate of basic mercury(II) amido-iodine is formed.

$\mathrm{2K_{2}HgI_{4}\: +\: NH_{3}\: +\: KOH\: \rightarrow \: HgO.Hg(NH_{2})I\: +\: 7KI\: +\: 2H_{2}O}$

For the analysis of cations belonging to groups I-VI, the cations are precipitated from the original solution by using the group reagents according to the scheme shown in the flow chart given below: The separation of all the six groups is represented as below:

Group reagents for precipitating ions:

Group	Cations	Group Reagent
Group zero	$NH{_{4}}^{+}$	None
Group-I	$Pb^{2+}$	Dilute $HCl$
Group-II	$Pb^{2+},Cu^{2+},As^{3+}$	$H_2S$ gas in presence of dil. $HCl$
Group-III	$Al^{3+},Fe^{3+}$	$NH_4OH$ in presence of $NH_4Cl$
Group-IV	$Co^{2+},Ni^{2+},Mn^{2+},Zn^{2+}$	$H_2S$ in presence of $NH_4OH$
Group-V	$Ba^{2+},Sr^{2+},Ca^{2+}$	$(NH_4)_2CO_3$ in presence of $NH_4OH$
Group-VI	$Mg^{2+}$	None